[llvm] [TableGen][DecoderEmitter] Add option to emit type-specialized `decodeToMCInst` (PR #146593)

[Rahul Joshi via llvm-commits]

jurahul wrote:

Right, this is POC at this which shows that the proposed optimization works. I am open to changing the interface here as well. The command line one was simple enough to not mess with tablegen instruction class etc, but that is an option, though it feels more intrusive. The command line is moderately complex and localized to the decoder emitter. 

https://github.com/llvm/llvm-project/pull/146593